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Sea Level Rise Modeling
Map of North Carolina coastal area showing Albermarle, Pamlico and other small sounds with adjoining rivers. 

Figure 1.  North Carolina sound system

A model to examine the impacts of long term sea level rise (SLR) is being implemented in the coastal North Carolina ecosystem. This area is particularly vulnerable to SLR, as a fragile system of barrier islands protects an extensive but sensitive estuarine system. The primary impact of SLR is on the hydrodynamic response of the system: circulation, tidal amplitude, and inundation patterns due to tides, winds, and storms can all change in response to rising sea level. Rates of SLR in the region are nearly 3 mm/year and are increasing; furthermore, inundation is tied to inlet conveyance which can be modified by SLR. A two-dimensional hydrodynamic model is being used to simulate tidal response, regional synoptic wind events, and hurricane storm surge propagation to study changes due to SLR. Accurate simulation of inundation patterns is accomplished by high localized resolution in the coastal zone, continuous bathy/topo data, and an accurate wetting/drying algorithm. The model will be validated against observational data before modification of initial and boundary water levels to represent eustatic SLR. Shoreline migration can be dynamically computed from the model simulation output as a function of SLR. Finally, the hydrodynamic model will be coupled to submodels that characterize the ecological impact of SLR. This work comprises the ´Ecological Effects of Sea Level Rise´ project being led by NOS´ National Centers for Coastal Ocean Science.

Model Development
The Coast Survey Development Lab has implemented a hydrodynamic model of the Pamlico/Albemarle sound system of North Carolina. The two-dimensional version of the ADCIRC finite element model is used. A triangular grid was created to cover the entire domain and a water level time series was produced at each node in the grid. The semidiurnal tidal high water and low water marks were extracted from the modeled time series and used to calculate tidal datums (e.g., Mean High Water (MHW), Mean Lower Low Water (MLLW)). The calculated tidal datums were compared to NOS water level station data at locations throughout the domain. The model results were adjusted to match the station data at those locations by spatially interpolating the error, so the corrected model results match the published NOS datum information in the region. The final tidal datum results were used to populate regularly spaced grids that were created as a component to the VDatum software. The VDatum tool allows the transformation between ellipsoidal, orthometric, and tidal datums.  After the VDatum tool was created, it was used to transform the bathymetry data in the region to the North American Vertical Datum of 1988 (NAVD 88). The adjusted bathymetry was combined with topographic LIght Detection and Ranging (lidar) data (also referenced to NAVD 88) to create a seamless elevation field. A 6 m horizontal resolution continuous bathymetric/topographic (bathy/topo) Digital Elevation Model (DEM) was constructed for accurate modeling of inundation. The final DEM covers a subset of the VDatum region with the focus at Beaufort, NC.

A Coastal Flooding Model (CFM) has been developed for the region by combining the tidal finite element hydrodynamic model with the continuous bathymetric and topographic elevation dataset. The CFM domain extends from 90 km offshore of the Outer Banks to the 15 m topographic contour and from northern Currituck Sound south to the New River. The CFM provides high resolution of coastal features down to 50 m. The CFM is relative to the NAVD 88 vertical datum and is populated with DEM elevations where available and other topographic and bathymetric data relative to NAVD 88 elsewhere to create a continuous bathy/topo elevation field.

 Graphic showing construction of the continuous bathy/topo DEM by transforming NOAA bathymetry with the VDatum tool and combining it with North Carolina FEMA LIDAR topography.

 Figure 2. Construction of the continuous bathy/topo DEM

Model Application
The CFM will model different scenarios of sea level rise as well as inundation from high intensity storms that sweep through the region. First, changes in tidal harmonic constants can be calculated under the effect of SLR. Second, changes in tidal datum shorelines can be calculated throughout the study area as demonstrated for the local test region. For example, the Figure 3. shows a nautical chart of the Morehead City and Beaufort, North Carolina area. The dark black line shows the charted MHW shoreline. The red line shows the modeled present day MHW shoreline. The green line shows the modeled MHW shoreline with a 30 cm rise in sea level. Third, the impact of synoptic wind events can be examined by forcing the CFM with wind fields and validating with water level records. This is an important process in the North Carolina sounds since much of the system is non-tidal and the primary inundation events are wind-driven, such as northeasters. The range and extent of inundation will be impacted by SLR, which can be shown by model output. Fourth, the CFM can be utilized to study hurricane storm surge flooding of the NC system and the significance of changes in flooding with SLR. Fifth, the model output will provide input to other models being developed for the project.

The development of the CFM has been to support the Ecological Effects of SLR study. Using the CFM to study changes in inundation with SLR is not complete without considering ecological processes, including erosion and deposition; marsh evolution; productivity of oyster reef, submerged aquatic vegetation, and benthic habitats; and anthropomorphic change. Therefore, the CFM will be used to drive a suite of ecological submodels of these processes. These submodels and the CFM can provide iterative updates to each other to generate an overall prediction of the ecological effects of SLR. Included in these ecological submodels are the impacts of anthropomorphic changes such as shoreline hardening in response to SLR; this can provide coastal managers with key modeling and mapping tools to assess the risk of SLR to the NC coastal environment.

 Nautical chart of area near North Carolina that shows color coded predicted shoreline change due to sea level rise by use of the Coastal Flooding Model.
Figure 3.  Predicted shoreline change due to sea level rise by use of the Coastal Flooding Model.
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